The arch of an underpass is a semi - ellipse 60ft wide and 20 ft high. find the clearance at the edge of a lane if the edge is 20ft from the middle.

Backofbook:

Don't I love ellipses.

So the ellipse is horizontal. Its major axis, 2a, is 60 ft. Its minor axis, 2b, is 2*20 ft.
If the center of the ellipse is the center of the lane, then the ellipse is
(x^2)/(a^2) +(y^2)/(b^2) = 1
(x^2)/(30^2) +(y^2)/(20^2) = 1 -----(i)